child brides in China
China: Too many men, not enough women. Men snap pictures of a dancer performing at a car show in Tianjin, China. Some Chinese men turn to child brides from Vietnam because they're the only wives they can find and afford Reuters/Stringer

Hundreds of child brides, some as young as 13, are being trafficked from Vietnam to marry Chinese men who can't find wives because of China's gender imbalance.

China's one-child policy combined with the long-standing preference for sons has heavily skewed the nation's gender ratio. By 2020, the National State Population and Family Planning Commission projects that males of marrying age in China will outnumber females by at least 30 million.

Now the villages along the Vietnamese-Chinese border have become a hunting ground for human traffickers to provide wives to single men. Girls are tricked or drugged, then sneaked across the border by boat, motorbike or car, reports CNN.

"It costs a very huge amount of money" for a Chinese man to marry a local woman in part because a number of men are competing for her hand in marriage, said Ha Thi Van Khanh, national project coordinator for the United Nation's anti-trafficking organization in Vietnam.

Traditionally, Chinese men wishing to marry local women are expected to pay for an elaborate banquet and to have purchased a new home, so "this is why they try to import women from neighboring countries, including Vietnam," she said.

Vietnamese brides can sell for $3,000 (£2,040). They're considered particularly desirable because of their cultural similarities to the Chinese. In one case tracked by CNN reporters, a 16-year-old girl was drugged and smuggled into China, where she was pressured to marry. When she refused she was beaten and starved until she relented.

She said her husband was kind to her, but she never stopped missing her family. "My desire to go home was indescribable," said the girl, Nguyen. "I agreed to marry the man but I could not stay with a stranger without any feelings for him."

When her mother-in-law realized she was never going to warm to the marriage, the family returned her to the traffickers. They got their money back, and Nguyen she was forced into a second marriage. She finally fled back home but had to leave her child behind.

Chinese and Vietnamese officials are working together to help stem the trafficking problem, and some organisations like the Pacific Links Foundation run a shelter for trafficking victims in Vietnam.

Pacific Links takes victims each week to a market in northern Vietnam where they talk about their experiences to people there. "I think awareness is the only tool," to end trafficking, said organisation founder Diep Vuong.